If you're looking to identify an argument as deductive or inductive, valid or invalid, strong
or weak, sound or unsound, cogent or uncogent— or better yet— if you want to understand the big
picture of how all these logical concepts relate to each other... you've come to the right place
The first thing to ask is, "What is the aim of the argument?" Some arguments try
to make their conclusion guaranteed. That's the hallmark of a deductive argument. For example,
all squirrels are mammals. This is a squirrel. So this is a mammal. This argument isn't trying
to show that this is probably a mammal... that would be weird! It's trying to show that clearly
this must be a mammal. It's a deductive argument. Its goal is a guaranteed conclusion.
But some arguments only aim to make the conclusion probable. These are inductive
arguments. For example, grizzly bears have not been seen in these parts for many years.
So we should be safe from Grizzlies while hiking on this trail. This is pretty good reasoning,
but the conclusion is not a sure thing. And that's okay. It's an inductive argument, so its goal is a
probable conclusion. So here in this first level of our chart the difference between deductive and
inductive reasoning is all about the aim of the reasoning. It's what the argument is trying to do
that makes it deductive or inductive. That's important because here in the second level of
the chart, the question is not what the argument is aiming to do but whether the reasoning succeeds
in what it's aiming to do. Let's start here on the deductive side.
Assume for a moment the premises are true, then is the conclusion guaranteed to be true?
If not, the argument is invalid. That means the reasoning went wrong somewhere. For example,
Mary's pet is black and white. Penguins are black and white. So Mary has a pet penguin. Well... no!
Even if we assume the premises are true, that doesn't mean the conclusion
has to be true. For instance, it might be that Mary has a black and white cat.
In that case, the premises are true — Mary has a black and white pet and penguins are still black
and white — but the conclusion would be false. So this is invalid reasoning. Because the premises,
even if true, do not guarantee the conclusion. But sometimes when you assume the premises are true
you can just see that the conclusion therefore has to be true. This is valid reasoning.
For instance, this puppy is either a Schnauzer or a bulldog. It's not a Schnauzer. So it's a
bulldog. Now if it really were true this puppy is either a Schnauzer or a bulldog,
and if it really were true that it's not a Schnauzer, well... then it would just
have to be a bulldog. How could it be otherwise? This argument is valid. It succeeds in its aim
of a guaranteed conclusion. This same pattern repeats itself over here on the inductive side,
only now the requirement for success has a lower bar. Assume for a moment the premises are true.
Then is the conclusion probably true? If not, we say the argument is weak. For example, I spotted
a Kestrel last weekend while out bird watching. So we'll probably spot one this weekend, too.
This premise, all by itself, just doesn't provide enough information to make it likely we'll see a
Kestrel. It's a weak argument. It fails at its aim of making the conclusion probable.
But sometimes when you assume the premises are true, it's clear that,
yes, the conclusion would be probable. In that case, we call the reasoning strong.
For example, the jaguar is sally's favorite animal. She also enjoys
wildlife photography.So she'll probably enjoy this photo of a jaguar. Of course, it's at least
possible that Sally won't like this photo. Maybe she's tired of jaguar photography or maybe there's
just something about the look of this particular jaguar that she just doesn't like. That could be.
But still, it seems pretty reasonable to think that probably she'll enjoy this photo. After
all she enjoys wildlife photography and this is her favorite animal. So it's a strong argument.
It succeeds in its aim of a probable conclusion. So far, we've been assuming the premises are true.
And that was fine because we don't need to know whether the premises are true to evaluate whether
an argument is valid or invalid, strong or weak. For example, who is Sally anyway? And is it really
true she likes jaguars? And wildlife photography? Nobody knows! Because I just made all this up.
But notice: that didn't stop us from recognizing this as strong reasoning. Back to the flow chart.
You might be thinking: I still don't see how we can just assume the premises are true. Doesn't
truth or falsity matter? The answer is, No. At least not here in this section of the flow chart.
We're trying to determine whether the reasoning is any good: valid or invalid,
strong or weak. And for that, we don't need to know whether the premises are actually true.
Here's one more example. All Shar Peis eat bananas. All banana eaters can fly.
So Shar Peis can fly. Now all of these claims are ridiculous. But notice something interesting. If
these premises were true — and they're not — but if they were, then this conclusion
would have to be true. In other words, this is a valid argument. There's absolutely nothing
wrong with the reasoning. But there are two ways an argument can go wrong. First,
the reasoning can fail. The premises might fail in their aim of supporting the conclusion.
Remember, we call this weak or invalid reasoning. But second, even if the reasoning is flawless,
the premises might be false. In that case, you are reasoning flawlessly... about falsehoods!
That's what happened here. It's perfect reasoning about nonsense. So back to the flowchart.
Here in this layer, we're checking the reasoning. But as we've seen, there is more to an argument
than just the reasoning. We also need to check whether the premises are actually true.
So if an argument is valid, we need to keep going and ask, "Yeah, but our premises actually true?"
If not, the argument is unsound. And that's how to classify this argument. It's valid but
unsound. Other times a valid argument will have true premises. These are what we call
sound arguments. We saw one earlier here. The premises are true and the reasoning is valid,
so it's a sound argument. Once again, everything we've just said is reflected equally over here
on the inductive side. It's not enough for an argument to be strong. We have to also ask,
"Yeah, but are the premises actually true?" If not, the argument is uncogent — which to my ear
is an ugly word— so I usually just say not cogent. But call it what you will. Our jaguar argument
was not cogent. Yes, it has strong reasoning but since the premises were just made up, we can't say
they're true. So strong but not cogent. However, if a strong argument does have true premises,
then it's cogent. This bear argument we saw earlier is cogent because the reasoning is strong
and I publish these videos from California, where I do lots of hiking and where the last Grizzly
was spotted way back in 1924. So this premise is actually true, making it a cogent argument.
So there you have it: the master logic flow chart.
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And if you're looking for more like this, check out my playlist on the fundamentals of logic.